This invention concerns a piezoelectric transducer comprising a combination of sensitive coaxial elements.
A piezoelectric transducer can be used, for example, for the measurement of pressures, especially in the fields of oceanography or seismic prospection or even for the measurement of directional stresses.
Piezoelectric transducers generally comprise one or or more sensitive elements each including a substrate possessing piezoelectric properties and two electrodes or armourings disposed on both sides.
The substrate may be produced in the form of ceramic disks with relatively small dimensions and included inside a rigid box provided with one or more sides transparent to acoustic waves. French Pat. No. 2.122.675 provides a piezoelectric sensor where one, and, preferably, two ceramic disks, each provided with electrodes, are secured to two flexible plates opposite each other. At least one of the flexible plates is provided with a thicker rigid peripheral part supported against the other. The disks may be produced by the sintering of powdered materials possessing piezoelectric properties and are extremely sensitive but mechanically fragile.
The substrate may be produced in the form of flexible strips or plates made from a plastic synthetic material upon which a treatment confers piezoelectric properties. The materials generally used are, for example, PVDF (polyvinylidene fluoride), polyethylene, PTFE (polytetrafluorethylene) etc, which are stretched prior to an electric polarization being applied to them.
As specialists already know, it is also possible to use copolymers which crystallize directly in a polarizable form without them firstly needing to be stretched.
By using flexible plates or strips, it is possible to build relatively long continuous hydrophones which produce wave number filterings. Incorporated inside seismic streamers towed by a vessel, they particularly allow for the filtering of certain radio disturbances and facilitate the receiving of signals reflected by discontinuities from the submarine subsurface in response to seismic waves emitted by a towed vessel.
Piezoelectric transducers have already been produced having a coaxial form by using recognized techniques in the field of producing signal transmission cables. The dielectric material disposed between the internal and external armouring is then replaced by a substrate made from a synthetic plastic material, such as those mentioned above, and are radially polarized. The external armouring is sufficiently fine so as to be transparent to acoustic waves. For example, a film of conductive paint or vacuum metallization is used. An impervious elastomer sheath electrically insulates the external armouring. This external armouring and the internal armouring are connected to an impedance corrector preamplifier. As conventional production techniques are used, the cost of coaxial transducers is often less expensive than the price of thin and flat film transducers. Moreover, the outer armouring constitutes a natural electric shielding.
Such coaxial piezoelectric transducers are described, for example, in the U.S. Pat. No. 4,568,851 or the published PCT patent application WO 86/00.757.
Piezoelectric coaxial cables are often unsuitable for use in water when they are subjected to various disturbances. This applies when they are used as hydrophones and towed at sea for the requirements of seismic prospection or for carrying out markings and localizations of sound sources. A radial acceleration provokes an over pressure on one side of the cable and a partial depression vacuum on the other hand. Current charges are generated due to pressure variations. These are of the opposite kind on both sides of the cable, but generally, there is no full compensation. An acceleration thus creates a parasitic voltage. If such a piezoelectric cable undergoes bending or flexion when being towed, compression of the sensitive substrate will be produced on the convex side and an extension on the opposite side. The charges of opposed signs which appear generally no longer compensate for each other. Like radial sensitivity, the parasitic signals generated from accelerations or flexions may even mask the useful signals to be measured.
The piezoelectric transducer according to the invention enables the above-mentioned drawbacks to be avoided.
The piezoelectric transducer of the present invention is produced from sensitive elements with a coaxial structure comprising a substrate including a material possessing piezoelectric properties, each connected to two conductive armourings, one being internal, the other external. These sensitive elements are respectively produced from radially polarized substrates but both being in an opposite direction and their armourings are electrically connected two by two. In accordance with advantageous features of the present invention, a piezoelectric transducer is provided which comprises at least two sensitive elements with substantially identical dimensions disposed close to each other and linked together by a covering material.
The outer armouring of one of the sensitive elements is connected preferably to the inner armouring of the other sensitive element and vice versa. An even number of sensitive elements whose radial polarizations are opposite two by two can be distributed into rings and embedded inside a covering material. They may even be disposed inside a sheath provided with a central recess being used, for example, to have various cables or possibly pipes run through.
The use of sensitive elements with substantially identical dimensions, which, owing to this fact, generate analogous voltages in response to a given stress, are thus able to willingly compensate for the accelerations or flexions which may be applied to them following possibly quite different directions.
Thus, by multiplying the number of sensitive elements, it is possible to obtain a piezoelectric transducer able to effectively compensate for radial flexions or accelerations, irrespective of their application directions. But on the other hand, it is thus possible, by licensing a particular direction, to obtain a piezoelectric transducer able to measure directional stresses.
Other characteristics and the advantages of the transducer according to the invention shall be revealed from reading the description of several embodiments, given by way of examples, which are by no means restrictive, and by referring to the accompanying drawings.